Reactive sputtering process

ABSTRACT

In magnetron-type reactive sputtering the properties of the deposited layer  are to remain constant throughout the entire use of a target,  independey of the state of erosion, even after an exchange of targets. The method is also to be applicable for magnetron sputtering sources having a target consisting of several components with different partial discharge powers. 
     Before sputtering of the substrates, the magnetic field strength associated with each partial target is set without reactive gas. Thereafter, a predetermined set of values of characteristic parameters is set by control of the reactive gas flow. During the subsequent sputtering the set of values predetermined for each partial target is kept constant by the controllable reactive gas flow. The first two steps are repeated at certain intervals in dependence of time the targets are used. 
     Optical coatings or corrosion protection coatings may be fabricated by reactive sputtering in accordance with this method.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of reactive coating of substrates in asputtering apparatus operating on the magnetron principle. For instance,coatings having optical properties or corrosion protection coatings maybe fabricated by this method.

In reactive coating, an electrically conductive target, preferably Al,Ti, In, C, is sputtered. The introduction of reactive gases such as O₂,N₂, H₂, C_(x) H_(y) and others into the discharge chamber leads to theformation of such compounds as Al₂ O₃, SiO₂, TiN, for instance.

By guiding the discharge plasma in an integrally enclosed magnetictunnel formed by arcuate curved magnetic field lines in front of thetarget, the density of the plasma and the sputtering rate are increased.The reactive magnetron discharge is determined, among others, byparameters depending upon the target material and upon the reaction gas.These parameters are the discharge current intensity and the dischargevoltage which essentially define the operating point of the process. Onaccount of the reactive gas component the magnetron discharge isprovided with an additional degree of freedom. With a great many targetreactive gas combinations instabilities occur at least in portions ofthe operating range. The discharge moves to a completely differentoperating point having different parameters and, therefore, othercoating properties.

2. The Prior Art

For stabilizing the reactive magnetron discharge, it is known to selectthe partial pressures of the inert gas and the reactive gas such thatthe discharge current or the discharge voltage remains constant at achanging reactive gas pressure and to utilize the dependence of thedischarge current or discharge voltage from the partial pressure of thereactive gas for controlling the reactive gas (DD 146,306). Furthermore,an analogous control process is known by which the operating point atthe associated reactive gas flow is calibrated and maintained constantby metering of the introduced reactive gas (DE 4,106,513 A1). Theseprocesses suffer from the disadvantage of being suitable only for arelatively brief stabilization for a fraction of the target use time,since during the target use time the effective magnetic field strengthand, hence, the discharge parameters and the reaction kinetics change asthe depth of erosion increases, which leads to changed properties of thedeposited layers. For this reason, the operating point requires constantadjustment in response to measured layer characteristics. Furthermore,these methods are of use only in connection with magnetron sputteringsources having but one discharge plasma. Furthermore, a reversesputtering zone of increasing width is formed at the target edges as aresult of the focussing effect of the erosion moat. Particularly withinsulating layers such as Al₂ O₃ or SiO₂, these reverse sputtering zoneslead to arc discharges and, hence, to interfering particles in thedeposited layer. A further significant disadvantage of the currentlyknown solutions is that they do not allow the realization of a reactivesputtering process by several partial targets of the same or differentpartial output which is stable over a long period and is reproducibleover the time of use of a target.

For depositing layers on substrates having a diameter in excess of 100mm--with layer thicknesses and layer properties of very goodhomogeneity--it is also known to separate the target into severalpartial targets. In such a magnetron sputtering source, also known as adual ring source, two partial targets are concentrically arranged with adischarge plasma burning in front of each partial target (DE 4,127,262C1). The output density and, hence, the target erosion on the concentricpartial targets differ from each other by a factor of 2 to 5, dependingon the coating geometry. Therefore, the change of the plasma parametersdiffers significantly with increasing target erosion. In a non-reactiveoperation, it is possible and sufficient to set the uniformity of thecoating thickness by selection of partial discharge outputs. A greatdisadvantage of the reactive operating mode of such a magnetronsputtering source resides in the excessively high demands put on theprocess to obtain a homogeneity of the stoichiometry which is stableover a long time and which is reproducible. The reasons for this arethat the stability of the discharge, its operating point and, therefore,its stoichiometry are decisively determined by the ratio of outputdensity to the supply of reactive gas. Some layer properties (e.g., thehardness) and the rate of deposit depend, aside from the stoichiometry,also upon the set values of the discharge voltage and/or the dischargecurrent {P. Frach et al., Surface and Coating Technology, 59(1993) 177].If only the partial discharge output is stabilized, it will in the endresult in a drift of the coating properties and its homogeneity. Thus,for example, a brief localized variation in the flow of oxygen in thereactive sputtering of Al₂ O₃, from the Al target, leads to a shift ofthe operating point of the discharge of the given partial target and,hence, to changed properties.

Furthermore, it is known to carry out reactive magnetron discharge byapplying a pulsed voltage (alternating voltage) to two targets ofdifferent materials. The process parameters of discharge current andvoltage and the gas pressure are measured by sensors and compared with adesired value. To obtain identical sputtering velocities of bothtargets, the signals obtained are processed in a control program toyield qualitatively good layer properties on the substrate (DE4,324,683). The process suffers, however, from the fact that it iseither not possible or difficult to control the discharge output oroutput density of the individual targets or partial targetsindependently of each other. The reason for this is that the requiredparameters for comparison with the desired value cannot be detected foreach target or partial target.

TASK OF THE INVENTION

It is the task of the invention to maintain constant the properties ofthe deposited layer, such as chemical composition, physical propertiesduring the coating of a substrate as well as during the entire time ofuse of a target, independently of the state of erosion. Even afterchanging a target, a renewed adjustment of the process, i.e. calibrationor constant correction of the operating point in accordance withobtained layer properties, is to be avoided. The definition ofcharacteristic layer properties is to serve only for a final qualitycontrol. The reverse sputtering zones are to be avoided or theirspreading is to be reduced. The method is to be suitable for differenttypes of magnetron sputtering sources with one target or a targetconsisting of several parts. The method is also to be applicable atdifferent partial discharge power or power density for the partialtargets. Also, the method is to make it possible rapidly to fabricatefrom electrically poorly conductive or insulating compounds such as Al₂O₃, SiO₂ layers of permanent properties and low particle density in thedeposited layer.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the invention provides for a method of reactively coatingsubstrates by means of a sputtering source operating on the magnetronprinciple and comprising a target made up of at least one component, anadjustable magnet system for generating magnetic fields of selectivelyvariable strengths, at least one controllable gas inlet and a source ofpulsating power, the method being practiced by a first step of initiallyestablishing, prior to sputtering of the substrates, a set of values ofparameters characteristic of a non-reactive plasma by adjusting, in anatmosphere of inert gas, at least one of the magnetic field strengthassociated with each partial target and the feed time of the pulsatingpower, by a second step of establishing for each partial target a set ofvalues of parameters characteristic of the reactive discharge bychanging the discharge parameters while maintaining constant the setvalues of magnetic field strength by at least one of admission of one ofreactive gas and a mixture of reactive and inert gases and by feedingpulsating power, by a third step of maintaining constant, duringsputtering of the substrates, said predetermined set of values for atleast a fraction of use of each partial target by controlling at leastone of the gas flow and gas pressure and the power feed time, and byrepeating said first and second steps in predetermined intervals beforecontinuing sputtering.

Practicing the method in accordance with the invention demonstrates thatit is possible to monitor and characterize the ratio of the plasmaparameters to the reactive gas necessary for the reactive forming of alayer, solely by electrical measured values, and that the sputteringprocess may be controlled in a manner which is stable over an extendedperiod and reproducible, by means of control circuits by setting ofsuitable determinative parameters, such as the magnetic field, the inputtime of the power supply and the flow of reactive gas. It is alsopossible optically and/or electrically to monitor the parameterscharacteristic of the plasma discharge. It is advantageous to utilizethe ratio of the optical intensities of two characteristic spectrallines of the plasma discharge as an optical parameter. It is possible bythis method to carry out the reactive deposition process over the timeof use of a target and from one target to another, with the propertiesof the layers remaining the same. Therefore, constant monitoring of theproperties of the layers and adjustment of the process parameters on thebasis of the results of such monitoring is not necessary for an extendedprocess operation. In this manner, the rate of rejects of the coatedsubstrates is significantly reduced and automation leads to higherproductivity and reduced costs.

By setting the individual magnetic field strength and/or the individualpower feed time for each target it is possible to maintain the plasmaparameters constant during the target erosion period free of theinfluence of the reactive gas. An individual pair of values, e.g. ofdischarge voltage and discharge current, at an average power over theduration of the period, is utilized as a measure for the setting. Theseelectrical parameters may be kept constant during the target erosion; orthey may also be determined by a single previously performed setting,e.g. for an Al target, in dependence of the target erosion and then usedas control values for further targets made of the same material. Anotherpositive effect is that target erosion occurs more uniformly and thatthe progressive formation of reverse sputtering zones is permanentlyavoided or strongly reduced. In this manner the tendency to arcdischarges and the formation of particles in the deposited layer isavoided or drastically reduced.

No change in the magnetic field strength occurs during the reactivecoating operation. An individual pair of values, e.g. of dischargevoltage and discharge current, at an individual power are averaged overthe duration of the period. These electrical parameters may be keptconstant during the target erosion, or they may also be determined by apreviously performed setting in dependence of the target erosion andthen be used as control values for further targets made of the samematerial.

If sputtering is to take place during an unchangeable power input time,the setting and stabilizing of the operating point of the reactivedischarge is performed by setting the flow of the reactive gas by meansof a reactive gas control loop.

When practicing the method with a common reactive gas inlet for severalpartial targets it may advantageously be controlled by a set of valuesof parameters characteristic of the reactive discharge of a selectedpartial target, e.g. the one having the largest partial dischargeoutput. For further partial targets, the operating point of the reactivedischarge is set and stabilized by setting the individual power inputtime by means of a control loop. However, this method is equallysuitable for magnetron sputtering sources with targets consisting of onepart only.

The magnetic field may be generated by permanent magnets as well as byelectromagnets or a combination thereof. A change in the magnetic fieldstrength may be obtained either by mechanically adjusting the spacing ofa permanent magnet system from the surface of the target and/or by achange of the current flowing in the coil of an electromagnet.

The duration of the period of intermittent feeding of power into theplasma may advantageously be in the range of several micro seconds up toseveral seconds. Period durations especially in the range of 5 microseconds to 50 micro seconds result in an operation of particularly fewarc discharges.

A further reduction in the tendency to arc discharges may be obtained bya low ohmic conductive connection between partial target and anodepotential, with the voltage between the partial target and the anodebeing preferably less the 15 V. The use of an arc recognition circuit inthe connection between the partial target and the anode of the dischargefor switching off the input power and subsequent low ohmic conductiveconnection result in a reduction of power fed into the arc so that aninfusion of particles into the deposited layer is prevented orsignificantly reduced.

For practicing the method it is of advantage to input power in such away that during the input time the partial targets always act ascathodes of the discharge. Discharge occurs against a further electrodeset as an anode which may be electrically insulated from the vacuumchamber, electrically connected with it or be a component of it.

If two partial targets are used, a further advantageous embodiment ofthe method includes one partial target being a cathode and the secondpartial target being an anode of the discharge. In the ensuing periodthe other target is either the cathode or the anode of the discharge.The alternation in the poling of the partial targets should be such thateach partial target regularly functions once as an anode during eachperiod in order to provide for cleaning of the partial target atpredetermined intervals. The power input during the input times areindependently selectable for both pole directions. If desired, at leastone additional electrode may be set to an arbitrary electric potentialduring all periods. This arbitrary potential may, for instance, be ananode potential which has a favorable effect on the ignition behavior.In certain cases it may be useful to arrange at least one additionalelectrode as electrically floating. This has a stabilizing effect on thedischarge and reduces the arc discharge tendency.

In accordance with a further advantageous embodiment of the method theignition of the reactive discharge occurs in the inert gas--reactive gasmixture at a predetermined value of the reactive gas flow or partialpressure of the reactive gas and the operating point of the discharge isthereafter automatically set by the reactive gas control loop. In thismanner intermediate layers of undesirable compositions, of the kindformed when the discharge is ignited in inert gas, may be avoided. Insome combinations of target material and reactive gas the discharge inthe inert gas--reactive gas mixture and at low discharge voltages ortotal pressures, ignites substantially better. Moreover, in this mannerit is possible to achieve a coating without movement of a diaphragmbetween the magnetron sputtering source and the substrate. Such adiaphragm is required if the discharge is ignited in pure inert gasbecause a non-reactive layer would be formed until the operating pointhas been attained. On the other hand, this embodiment of the inventionoffers the advantage of the danger of the target being heavily coveredby reactive products, since in some reactive processes the setting ofthe operating point with the target being covered is possible only afteran unfavorably long sputtering time.

BRIEF DESCRIPTION OF THE SEVERAL DRAWINGS

The invention will be explained in more detail on the basis of twoembodiments. In the associated drawings:

FIG. 1: is a semi-section of a magnetron sputtering source in unipolaroperation;

FIG. 2: is a semi-section of a magnetron sputtering source in bipolaroperation.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The magnetron sputtering source of FIG. 1 consists of partial targets 1,1', which are affixed to cooling plates 2, 2'. The magnets 3, 3' of themagnet system arranged behind the partial targets 1, 1' may bemechanically moved in the direction of the partial targets 1, 1', as iswell known. A reactive gas inlet 4, 4' is provided on the side of eachpartial target 1, 1'. 5 and 5' designate control valves of the reactivegas inlet 4, 4'. Moreover, an additional electrode 6, a measurementsignal detection and control unit 7 and a power input unit 8 are part ofthis magnetron sputtering source. The power input unit serves to providea pulsating power for the partial targets 1, 1'. A substrate 10 ispositioned over the partial targets 1, 1' behind a pivotal diaphragm 9.11 designates an inert gas input arranged adjacent to the target.

In accordance with the invention, the method is practiced as follows: Ina first step, before the reactive sputtering of the substrates 10, themagnetic field strength associated with each partial target 1, 1' is setin inert gas (argon) without feeding of reactive gas. To this end, argonis fed in through an inert gas inlet 11. The control valves 5, 5' areclosed during this method step. At a predetermined power for eachpartial target 1, 1' the discharge burns between each partial target 1,1' and the additional electrode 6 set as an anode. At this time apredetermined discharge current and a predetermined discharge voltagewill be set for each partial target 1, 1'. By shifting the magnets 3, 3'associated with the partial targets 1, 1' the magnetic field strength ofeach partial target 1, 1' changes. This operation will continue untilthe predetermined characteristic parameters (discharge voltage anddischarge current) have been set at each partial target 1, 1'. Duringthis step, the pivotal diaphragm 9 shields the substrate 10 against thepartial targets 1, 1'. In a next step, the magnetic field strength ismaintained constant, i.e. the magnets 3, 3' are not moved. A constantflow of argon is set at the inert gas inlet 11. A predetermineddischarge voltage is set and maintained constant at the power supplyunit 8. Oxygen is fed through the reactive gas inlet 4, 4' so that thedischarge will now burn in a reactive atmosphere. During this time, apredetermined discharge current and, hence, a predetermined dischargepower will be set for each partial target. The given discharge currentis monitored in a known fashion at the measuring value monitoring andcontrol unit 7 and a signal is fed to the corresponding control valve 5,5' so that the flow of oxygen is thus changed until each discharge powercorresponds to the power predetermined for each partial target 1, 1'.During this step, the pivotal diaphragm 9 remains closed.

Sputtering of the substrate 10 is carried out during the next step. Tothis end, pulsating power is fed by the power supply 8 so that thedischarge is burning in an argon-oxygen atmosphere between each partialtarget 1, 1' and the additional electrode 6. During this time thedischarge currents of each partial target 1, 1' is monitored by themeasuring value monitoring and control unit 7, and the oxygen flow iscontrolled by the control valves 5, 5' in such a manner that thedischarge power of each partial target 1, 1' corresponds to the powerpredetermined for each partial target 1, 1'. Sputtering of the substrate10 is now carried out with the pivotal diaphragm being open.

In order to ensure constant discharge conditions and, hence, constantcoating properties, the first two steps of setting the magnetic fieldstrength for each partial target 1, 1' and of controlling the oxygenflow for each partial target 1, 1' for conforming the discharge power toa predetermined power (without substrate) are repeated after apredetermined number of coatings. Thereafter the sputtering of thesubstrates 10 continues.

In FIG. 2 there is also shown a magnetron sputtering source which isdifferent by the arrangement of only one reactive gas inlet 4 betweenthe partial targets 1, 1'. There is no additional electrode 6, sincethis magnetron sputtering source is operated in a bipolar manner, i.e.the partial targets 1, 1' are alternatingly switched to function ascathodes and anodes.

The first two steps of the method of setting the magnetic field strengthand the operating point are carried out as described in connection withFIG. 1. Since only one reactive gas inlet 4 is provided and sincebecause of the different geometry the discharges have differentparameters, the partial target 1 is first adjusted to a predeterminedpower by controlling the oxygen flow. The second partial target 1' thushas discharge parameters which deviate from a predetermined power. Inorder to adjust the discharge power of the second partial target 1' tothe predetermined power, the power supply time of the power supply ischanged until the measured power corresponds to the power predeterminedfor the second partial target 1'.

What is claimed is:
 1. A method of reactively coating substrates with amagnetron sputtering source including partial targets composed of atleast one component, a magnet system located behind each partial target,at least one controllable gas inlet, and a device that applies pulsedpower into a plasma, the pulsed power having a period composed of a feedtime and an intermission, the method comprising:(a) prior to coating ofthe substrate, changing at least one of a magnetic field strengthassociated with each partial target or the feed time of the pulsed powerin an inert gas without reactive gas at a discharge pressure, wherein aset of values of parameters characteristic of non-reactive plasma isobtained; (b) prior to coating the substrates in a constantly maintainedmagnetic field for the partial targets, changing discharge parameters byat least one of (i) admitting one of a reactive gas or a reactive gasand an inert gas, or (ii) changing the feed time of the pulsed power forthe partial targets, wherein a set of values of parameterscharacteristic of reactive discharge predetermined for each partialtarget is obtained; (c) during sputtering, maintaining constant thepredetermined set of values of parameters characteristic of reactivedischarge for each partial target for at least a fraction of a durationof use of the partial targets by at least one of (i) controlling one ofgas flow or gas pressure at the controllable gas inlet, or (ii)controlling a feed time of the pulsed power; (d) repeating (a) and (b)at predetermined intervals during use of the partial targets, andsubsequently continuing to coat the substrates.
 2. The method of claim1, further comprising forming the set of values of parameterscharacteristic of the plasma discharge from a value triplet of dischargevoltage, discharge current and power fed into the plasma measured over alength of the period.
 3. The method of claim 1, further comprisingforming the set of values of parameters characteristic of the plasmadischarge from a pair of values of discharge voltage and one of anoptically or electrically derived signal representing a state of theplasma.
 4. The method of claim 1, further comprising forming the set ofvalues of parameters characteristic of the plasma discharge from a pairof values of discharge power and one of an optically or electricallyderived signal representing a state of the plasma.
 5. The method ofclaim 1, further comprising forming the set of values of parameterscharacteristic of the plasma discharge from a pair of values ofdischarge current and one of an optically or electrically derived signalrepresenting a state of the plasma.
 6. The method of claim 1, furthercomprising determining the set of values of parameters characteristic ofthe plasma discharge in accordance with a state of erosion over theduration of use of the partial targets.
 7. The method of claim 6,wherein the formed set of values are one of the same or different in (a)or (b).
 8. The method of claim 1, wherein a length of the period is setin a range of several micro seconds to several seconds.
 9. The method ofclaim 1, wherein during the feed time, the partial targets are dischargecathodes, andwherein the discharge burns in a direction of at least oneadditional electrode provided in the discharge chamber, which is set asan anode.
 10. The method of claim 9, further comprising feeding powersuch that during a length of the intermission, the partial targets arelow-ohmically connected to an anode potential such that the voltagebetween the partial targets and the anode does not exceed 15 V.
 11. Themethod of claim 1, wherein, during a given period, the method furthercomprises setting some of the partial targets as cathodes and setting atleast one of the partial targets as an anode,wherein, during the feedtime of a next period, the method further comprises switching a partialtarget from a cathode to an anode and switching the at least one partialtarget from the anode to a cathode, and wherein, for both poledirections, the method further comprises independently selecting feedtimes and power fed during the feed times.
 12. The method of claim 11,wherein, during the periods, at least one additional electrode is set asan anode.
 13. The method of claim 11, wherein during the periods, atleast one additional electrode is set for an arbitrary electricpotential.
 14. The method of claim 1, further comprising determining anactual value of the supplied power used for controlling frommultiplication of discharge voltage and discharge current, andforming anaverage value during a given period.
 15. The method of claim 1, furthercomprising feeding one of the reactive gas or a reactive gas mixtureinto a discharge chamber adjacent to the partial targets.
 16. The methodof claim 1, further comprising igniting the discharge in inert gaswithout feeding of reactive gas; andautomatically setting the operatingpoint of the discharge by a reactive gas control loop.
 17. The method ofclaim 1, further comprising igniting the discharge in a mixture of inertgas and reactive gas; andautomatically setting the operating point ofthe discharge by a reactive gas control loop.
 18. The method of claim 1,wherein during reactive sputtering, the method further comprises feedingpower according to a constant power process; andsetting a reactive gasflow by a reactive gas control loop, whereby one of a discharge voltageor a discharge current is stabilized at a predetermined value.
 19. Themethod of claim 1, wherein during reactive sputtering, the methodfurther comprises feeding power according to a constant power process;andsetting a reactive gas flow by a reactive gas control loop, wherebyone of a discharge power or a discharge current is stabilized at apredetermined value.
 20. The method of claim 1, wherein during reactivesputtering, the method further comprises feeding power according to aconstant current process; andsetting a reactive gas flow by a reactivegas control loop, whereby one of a discharge voltage or a dischargepower is stabilized at a predetermined value.
 21. The method of claim 1,wherein, for some of the partial targets, reactive gas flow ismaintained constant, and for other partial targets, the feed time ofpulsed power is maintained constant,wherein the reactive gas flow andthe feed time of pulsed power are maintained constant by several controlloops on the basis of the predetermined set of values of the parameterscharacteristic of the reactive discharge predetermined for each partialtarget.